Stock-Based Compensation Plans - Restricted stock activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Awards Vesting Over Three Years [Member]
Number Outstanding
Vesting period	3 years
The 2007 plan
Number Outstanding
Balance at the beginning of the period (in shares)	525,356	515,130	682,654
Shares granted	146,440	237,708	151,408
Shares cancelled		(5,750)	(60,110)
Shares vested	(275,201)	(221,732)	(258,822)
Balance at the end of the period (in shares)	396,595	525,356	515,130
Intrinsic value of unvested awards (in dollars)	$ 7,131,787	$ 8,786,611	$ 7,060,474	$ 8,756,990
Stock options outstanding (in shares)	671,656
The 2007 plan | Restricted stock
Number Outstanding
Balance at the beginning of the period (in shares)	525,356	515,130	682,654
Shares cancelled		(5,750)	(60,110)
Shares vested	(275,201)	(221,732)	(258,822)
Balance at the end of the period (in shares)	396,595	525,356	515,130
Remaining average vesting period for recognition of unrecognized compensation expense	1 year 7 months 6 days
Unrecognized compensation expense (in dollars)	$ 4,700,000
The 2007 plan | Restricted stock | Awards vesting on first anniversary
Number Outstanding
Shares granted	16,440	50,208	21,408
The 2007 plan | Restricted stock | Awards vesting over four years
Number Outstanding
Shares granted	5,000	13,000	130,000
Vesting period	4 years	4 years	4 years
The 2007 plan | Restricted stock | Awards Vesting Over Three Years [Member]
Number Outstanding
Shares granted	125,000	174,500
1996 Plan
Number Outstanding
Stock options exercised (in shares)	49,000	26,437	54,991
Stock options outstanding (in shares)	0	49,000	75,437	136,928
Intrinsic value of outstanding stock options (in dollars)		$ 622,300	$ 601,991	$ 781,692
Vesting period	10 years